The present invention relates to a probe microscope such as an interatomic force microscope or magnetic force microscope of the type in which various kinds of forces, such as an interatomic force acting between substances, is converted into a displacement by means of a minute spring element and the displacement is detected by a photo-detector in that laser light is applied on the spring element and if there is any positional deviation of light reflected from the spring element, such positional deviation is indicative of the displacement, thereby producing a control signal.
The atomic force microscope as one kind of probe microscope has been expected to be a means for observing the surface configuration of a novel insulating substance and has now been studied since it was invented by G. Binnig as an inventor of STM (refer to Physical Review Letters vol. 56 p. 930, 1986). The principle of this microscope resides in that the interatomic force acting between a detecting chip having its top end sufficiently sharpened and a sample is measured as a displacement of a spring element attached with a detecting probe, the surface of the sample is scanned while the amount of displacement of the spring element is kept constant and the surface configuration of the sample is measured with a control signal for keeping constant the amount of displacement of the spring element serving as configuration information.
The spring element displacement detecting means is roughly classified into a STM system using a tunnel current and an optical system.
The STM system makes use of a so-called tunnel phenomenon, i.e. when a voltage is applied between two conductors which are held close to each other, leaving therebetween a distance in the range between several nanometers and several angstroms, a current begins to flow through the conductors. According to this system, the spring element is made conductive in advance, and a sharp metallurgical probe is caused to come as close as about 1 nanometer to the spring element and a tunneling current is made to flow through the spring element so that the resultant current value is used as a signal indicative of the displacement of the spring element.
As to the optical system, an example using a so-called interference method (refer to Journal of Vacuum Science Technology AG(2) p. 266 Mar./Apr. 1988) and an optical lever type probe microscope (Journal of Applied physics 65(1) p. 164 January 1989) are known. The optical lever type probe microscope utilizes an electric signal converted from an optical signal detected by an optical detecting element, the optical detecting element detecting a deviation of light reflected from a spring element irradiated with laser light, the deviation being due to the probe displacement.
The probe microscope to which the present invention relates is known as an optical lever type. The probe microscope is called an interatomic force microscope if it is of the type in which the probe arranged opposite to the sample is subject to an interatomic force from the latter while it is called a magnetic force microscope if it is subject to a magnetic force from the sample. Thus, it becomes possible to observe the state of the sample by detecting various kinds of forces emanating from, or associated with, the sample.
One known probe microscope is illustrated in FIG. 8. In this microscope, light emanating from a semiconductor laser 106 supplied with a signal from a laser driver 118 is focussed, or concentrated, on the rear surface of a spring element 2 through a lens 108 and the light reflected from the element 2 is focussed, or concentrated, on an optical detecting element 111 through a lens 109. When, for example, a two-element type photo-detector is used as element 111, if the photo-detector is so adjusted that light is incident uniformly upon the two pre-separated elements and the signal from each element is supplied to one input of a differential amplifier 19, so that a differential signal is received by differential amplifier 19, it is possible to observe displacement of the spring element 2 attached to a probe 102. The differential signal is supplied to a servo system 120 and in response thereto, a fine movement element 104 carrying sample 101 is driven in the z-direction, which is the vertical direction in FIG. 8. Then the drive signal in the z-direction is supplied to a computer 121 to produce an indication of the surface configuration of the sample.
However, in the case of the above-mentioned optical lever type interatomic microscope, since it drives the sample 101 by the fine movement element 104 fixed to a coarse movement mechanism 105, it has been usual that an attempt to observe a large sample results in lowering the resonance frequency of the fine movement element and so the observation becomes difficult. Further, where the fine movement element is or includes a piezoelectric element, since the diameter thereof is as small as about 30 mm. at the maximum, it is physically difficult to set up the sample. For example, in order to observe a semiconductor wafer or optical disk plate, it has been necessary to divide the sample, with the drawback that the advantage of nondestructive observation capability of the interatomic microscope cannot be fully utilized.
Further, the above-mentioned system has had the problem that since the sample 101 is driven by the fine movement element 104, the load mass on the element 104 fluctuates every measurement time so that the control characteristic and the measuring speed can not be kept constant.
Therefore, as shown in FIG. 9, a displacement detecting system formed of the spring element 2 and the photo-detecting element 111 is attached to the bottom of a fine movement element 104 and the sample 101 is fixed to the coarse movement mechanism 105. Further, in order to reduce the mass load on the fine movement element 104, the semiconductor laser 106 is arranged within a frame 114 and laser light is guided from a location above the fine movement element by means of an optical fiber 107. The spring element 2 is held against a support member 112 by means of a spring 113 and held stationary at a certain angle of inclination with respect to the optical axis of the lens 108. Light from the optical fiber 107 is converged on the top end of the spring element 2 through the lens 108 and light reflected from spring element becomes incident upon the two-element photodetecting element 111. Adjacent to the slight motion element there is provided a metallurgical microscope 115.
The coarse-moving mechanism 105 comprises a three axis stage for movement in x, y and z coordinate directions and the sample 101 is transferred between the metallurgical microscope 115 and the interatomic microscope. Thus there exists an interatomic microscope having the structure that a sample is coarsely observed in advance with a metallurgical microscope and then a part of the sample which is desired to be observed in more detail is observed with the interatomic microscope.
However, in the case of the interatomic microscope of the above-described structure, due to the fact that the displacement detecting system formed of the spring element 2, photodetecting element 111, etc., has its top end attached to the fine movement element, it is difficult to provide an alignment mechanism for converging a semiconductor laser beam on the top end of the spring element 2 because by so doing the weight supported by the fine movement mechanism increases. Therefore, it is not easy to align a laser beam with the top end of the spring element 2, which has a size of several of tens of .mu..